Lens-based high-resolution x-ray microscopy largely resulted from research work at synchrotron radiation facilities in Germany and the United States starting in the 1980s. While projection-type x-ray imaging systems with up to micrometer resolution have been widely used since the discovery of x-ray radiation, systems using x-ray lenses with sub-100 nanometer (nm) resolution began to enter the market only this century. These high-resolution microscopes are configured similarly to visible-light microscopes with an optical train typically including an x-ray source, condenser, objective lens, and detector.
Because x rays do not refract significantly in most materials, nearly all such high-resolution x-ray microscopes use diffractive objective lenses, called Fresnel zone plates, as objective lenses. Fresnel zone plates act as ideal thin lenses for monochromatic x-rays. They are essentially circular diffraction gratings, with the grating spacing decreasing with increasing distance from the center in order to progressively increase the diffraction angle and thus produce the focusing effect. By year 2009, x-ray microscopes using synchrotron x-ray sources have achieved 30 nm resolution, and commercial systems using laboratory x-ray sources have achieved 50 nm resolution.
Compared with the widely used visible light and electron microscopy techniques, x-ray microscopy combines properties that make it favorable for a large number of applications: (1) high energy x rays have a very large penetration length to image internal structures of thick samples without preprocessing; (2) the absorption and fluorescence emission depends strongly on the elemental composition of the sample, allowing high-sensitivity material analysis; and (3) x-ray imaging causes minimal structural damage to samples without inducing a charging effect upon the samples.
One key component of an x-ray microscope is the objective zone plate lens that focuses the x-rays and magnifies the transmitted image of the sample onto the x-ray detector. The diffraction-limited resolution of the zone plate lens is δ=1.22 Δrn, the focal length is f=2rn/(λΔrn), and the numerical aperture is NA=λ(2Δrn), where rn is the radius of the outermost zone, Δrn is the width of the outermost zone, and λ is the wavelength. Zone plates with zones intended primarily to block x-ray radiation are called amplitude zone plates. They can provide up to 10% focusing efficiency. Zone plates with zones intended to produce an ideally π phase shift are called phase zone plates. They can provide up to 40% efficiency. In practice, a zone plate will both absorb and phase shift the x-ray beam impinging on it, and will behave as a combination of an amplitude and a phase zone plate. For high-energy x-rays, the phase shift dominates and zone plates behave closer to phase zone plates. Even higher theoretical efficiency can be achieved when the zones approximate the profile of a Fresnel lens. This type of “blazed” zone plate can achieve 100% theoretical focusing efficiency, but is difficult to realize or approximate in practice.
The efficiency of a zone plate is limited in practice by the achievable thickness of the zones of the zone plates. An amplitude zone plate reaches its maximum efficiency when each zone completely absorbs the x-ray beam; and a phase zone plate reaches its maximum efficiency when each zone shifts the phase of the x-ray beam by π, with no absorption. For example, with higher x-ray energy, the zone thickness must be increased to maintain absorption or phase shift.
With higher energy x-ray radiation, thicker zone plates are required to achieve optimal efficiency. For example, a gold zone plate having a thickness of 1650 nm reaches a maximum efficiency of 31% at just below x-ray energy of 9.5 keV. At this same energy, a 350 nm thick zone plate has an efficiency below 3%, which illustrates that the efficiency of zone plates at higher x-ray energy values is limited by the thickness of the zone plates. Therefore, the main challenge when making high resolution and high efficiency zone plate lenses involves making zone plate structures with high zone plate thickness versus zone width aspect ratios, especially with increasing x-ray energy. For example, zone plates with a 50 nm outer zone width requires an aspect ratio of 33 to obtain optimum efficiency for an x-ray energy of 9.5 keV. Such a high aspect ratio often poses significant difficulty for fabricating a single optic element and has been a limiting factor in achieving high resolution imaging using higher energy x-rays.
The criticality in fabricating thicker zone plates is in the fabrication and the mechanical stabilization of the outer zones. It is here that the aspect ratios become extreme. This is because the outer zones are the narrowest zones, and yet also have to be the same height as the other, inner, wider zones. Fabricating these zones challenges existing fabrication processes such as plating technology due to the narrowness of the zones. In addition, because of their narrowness, the high aspect ratio zones are more susceptible to breakage by mechanical stress or other stresses due to charging effects.
Some have proposed to fabricate effectively thick zone plates by aligning and stacking separate zone plates to create a compound optic. One specific example relies on the formation of a zone plate doublet by fabricating two zone plates on either side of a common substrate. This approach is problematic, however, because it necessitates thin substrates and front side and backside alignment and fabrication. Moreover, the first fabricated zone plate must survive the fabrication process for the second zone plate. Another approach relies on the fabrication of a series of zone plates successively, stacked one on top of the other. In this approach, however, alignment tolerances increase with each stacked plate. As a result, the stacked approach requires effective planarization prior to forming the next zone plate of the stack, along with techniques for stabilizing the zones sufficiently to survive multiple planarization processes.
Nevertheless, compound x-ray optical elements have been developed. U.S. Pat. No. 6,917,472 B1 describes an Achromatic Fresnel Optic (AFO). This is typically a two element compound optic that is comprised of a diffractive Fresnel zone plate and a one or more refractive Fresnel lenses. Generally, AFO's have been proposed for imaging short wavelength radiation including extreme ultraviolet (EUV) and x-ray radiation. The diffractive element is the primary focusing element, and the refractive element typically provides no or very little net focusing effect. It serves to correct the chromatic aberration of the zone plate.